Three-dimensional interfacial networks in polycrystalline solids are topologically and crystallographically complex. With a few notable exceptions, most of what we know about the distribution of three-dimensional grain shapes has been derived from foams or computer simulations with isotropic boundary properties. We show here that the boundaries of individual grains in dense polycrystals prefer certain crystallographic habit planes within the polycrystal correspond to the same planes that dominate the external growth forms and equilibrium shapes of isolated crystals of the same phase. The observations reduce the external growth forms and equilibrium shapes of isolated crystals of the same phase. The observations reduce the apparent complexity of interfacial networks and suggest that the mechanisms of solid state grain growth may be analogous to conventional crystal growth. The conclusions are tantamount to a paradigm shift in our understanding of grain boundaries., which has been dominated for decades by consideration of crystal lattice orientation relationships instead of grain surface relationships.